Data transmission in long fiber transmission paths (such as undersea or transcontinental cable or lightwave transmission paths) are subject to signal fading and accompanying degradation in the signal-to-noise ratio (SNR) that are caused by effects of polarization. Signal fading and associated SNR degradation can also result from chromatic dispersion, material dispersion in the fiber, and polarization mode dispersion. Polarization is particularly described herein to demonstrate its effects on SNR and signal fading.
In a long lightwave transmission system with optical amplifiers, the SNR can fluctuate in a random manner due to various types of dispersion. Dispersion (such as the types of dispersion described above) causes delays in the data transmission channel, particularly in channels with long fiber-optic transmission paths. When the SNR of a signal in such a lightwave transmission system becomes unacceptably small, a signal fade has occurred.
Signal fading and the associated SNR fluctuations are caused by a number of polarization dependent effects induced by the optical fiber itself and other optical components (e.g., repeaters, amplifiers, etc.) along the optical fiber transmission path. In systems using optical amplifiers between the transmitter and the receiver, the gain from an amplifier is dependent on the state of polarization (SOP) of the lightwave entering the optical amplifier. Optical amplifiers with resynchronization capabilities reduce the effects of signal fading and address the delay problem due to long fiber-optic transmission paths. For optimal signal performance, the SOP of the optical amplifier matches that of the incoming lightwave so that a maximum possible gain is achieved at the output of the lightwave. The SOP of the lightwave is determined by the shape of the polarization ellipse, i.e., the direction of the major axis and the ratio of the major axis to the minor axis E.sub.ox /E.sub.oy, and the phase difference EQU Phase.sub.polarization =phase.sub.x -phase.sub.y
Random polarization fluctuations result because of random polarization phase changes or individual random amplitude change of polarization states, or both. In particular, signal fading due to polarization-dependent effects over long optical fiber transmission paths can be attributed to polarization-dependent loss (PDL), polarization-dependent gain (PDG), polarization mode dispersion (PMD) and polarization-dependent hole-burning (PDHB). All of these effects impact the SOP of an optical signal being propagated along the long optical fiber transmission path, and thus the effectiveness of optical amplifiers in offsetting signal fading and delay from the transmission medium.
A conventional solution to rectify the delay and signal fading problem in an optical channel is to simultaneously launch two signals of different wavelengths and substantially orthogonal relative polarizations into the same transmission path. Since the two signals are launched with equal power and orthogonal SOPs, the overall transmitted signal is essentially unpolarized. This has the advantage of reducing the deleterious effects of the transmission channel's non-linear signal-to-noise interactions, and signal delay caused by PDHB. Even though the average SNR performance improvement with such an arrangement can be substantial, such a system is still subject to significant signal fading and channel delay. Moreover, the two-wavelength source is still subject to signal fading. Moreover, it is costly and a waste of power to use two wavelengths to transmit data because only half the bandwidth required is needed to carry useful information. In addition, hardware resources such as dispersion compensation fiber, manual compensation tracking or variable dispersion compensation per channel in WDM, which may also be addressed to this problem, are expensive and burdensome to implement. In particular, a two-wavelength dispersion compensation technique for reduction of signal fading is practically unrealizable because information translation from one channel to another channel on a real-time basis is almost impossible to perform due to channel ranges and non-linearities in WDM.
Therefore, there exists a need for a simple, cost-effective single-wavelength dispersion compensation technique for reducing effects of optical signal fading, without using an additional channel. Furthermore, an automated dispersion compensation method is needed for reduction of signal fading on a real-time basis without optical signal power penalty and without prohibitive, additional retransmission adjustment methods. A dispersion compensation method is needed to rectify signal fading and SNR degradation due to various factors in optical transmission such as polarization, chromatic dispersion, material dispersion in the fiber and polarization mode dispersion.